Takuma Saito

In this study, we investigated the creep deformation mechanism of a single-crystal high-entropy superalloy (HESA) with the spherical γ′ precipitates at 760 °C. Before the creep tests, long-term aging tests at 760 °C without load were conducted, which showed Ostwald ripening of the secondary γ′ precipitates up to 50 h. The creep tests revealed that in the range of 500 and 600 MPa at 760 °C, the creep deformation mechanism of HESA was independent of applied stress in both the primary and secondary creep regions. The deformation mechanism of HESA was further investigated under the condition of 760 °C and 520 MPa by performing creep interrupted tests and microstructural analysis. Scanning electron microscope observation showed elongated γ′ precipitates along the applied stress axis near the ruptured surface. This could have been caused by the multi-slip around preceded by the lattice rotation into along the tensile axis, which was confirmed by the electron backscatter diffraction analysis. Transmission electron microscope observation of the creep interrupted and ruptured specimens showed bypass and climb motion of dislocations in the 2-h interrupted, shearing of the γ′ precipitates by the paired straight dislocations in the 50-h interrupted, and shearing of the γ′ precipitates by both the straight and the curved paired dislocations in the ruptured specimens, respectively. The secondary γ′ precipitates do not affect creep behavior as long as the deformation mechanism is a bypass and climb motion of dislocations.

The effect of solution heat treatment as well as primary and secondary aging treatment conditions on the microstructural evolution of a high-entropy superalloy was investigated. The as-cast sample shows coarsened γ′ precipitates and other extra phases at interdendrite region due to microsegregation. This microsegregation makes γ′ solvus unclear and decreases the solidus. After conducting the solution treatment determined in this study, primary aging conditions showing an aligned cubic γ′ phase at 1050 °C for 4 h and random spherical γ′ precipitates at 950 °C for 20 h in similar size were found. By using both samples, secondary aging conditions showing coarsened secondary γ′ precipitates and γ particles inside the γ′ precipitates at 800 °C for 20 h and fine secondary γ′ precipitates at 850 and 870 °C for 20 h were found.